Endoscopes have numerous uses and have proven particularly advantageous in minimally invasive surgery. By inserting an endoscope into a human body through a natural body opening or via a small surgical incision (or via a cannula inserted through the incised tissue), organs, joints, internal body cavities, and other body parts may be observed close up without the need for making large surgical openings. Typically, an endoscope is constructed with an elongate insertion portion having an optical system that comprises an optical objective lens unit adjacent the distal (front) end of the insertion portion, and an image relaying means behind and in alignment with the objective lens unit, and either an optical or electronic imaging means posterior to the image relaying means for providing a display of the viewed image. In electronic endoscopes, i.e., endoscopes using electronic rather than optical imaging means, the imaging means commonly comprises a solid state video sensor located in the insertion portion or adjacent to the proximal (rear) end of the insertion portion, with the sensor providing output signals that are used to generate a video signal input for driving a video monitor video monitor which is adapted to display the viewed image represented by the output signals of the video sensor.
Conventional monocular optical system endoscopes are limited to providing two dimensional images that lack any perspective of depth. Thus, although details of the viewed object may be quite clear in the horizontal and vertical directions, the lack of depth perception often brings confusion when the position of the viewed object is being judged relative to that of another object.
Stereo endoscopes eliminate or substantially reduce the depth visualization problem encountered with monocular optical system endoscopes. Conventional stereo endoscopes comprise two identical objective optical lens systems that are located side-by-side at the distal end of the insertion portion and are disposed so as to obtain the parallax required to realize a stereoscopic view, i.e., so as to capture two offset images of the same object from different angles. The two offset images are transmitted by separate image-relaying means to separate imaging means. The imaging means may be purely optical, comprising a binocular optical viewing assembly whereby the viewing person realizes a stereoscopic view of the viewed object from the two offset images. That kind of imaging arrangement is shown by U.S. Pat. No. 3,655,259 (see also FIG. 3 of U.S. Pat. No. 4,862,873). Alternatively the imaging means may comprise a pair of video sensors, with the output of the sensors being used to drive an external video monitor. The latter's input circuits are connected by video processing and control circuits to the two video sensors, whereby the stereo image pair represented by the output signals of the two video sensors cause the monitor to generate a stereo image display having the parallax of the two offset images. Observing and appreciating the stereoscopic image display provided by the monitor requires the use of specially designed 3D eyeglasses and involves an after-image phenomenon, as disclosed by U.S. Pat. No. 4,862,873. As a further alternative, the external video monitor may be replaced by a 3D headset in the form of spectacles that incorporate two separate video display means, one for each eye.
Unfortunately the advancement from single channel endoscopes to stereo endoscopes is complicated by the need to accommodate two like optical channels, since having two side-by-side optical channels appears to require, an increase in the outer diameter ("o.d.") of the insertion portion of the endoscope. The o.d. size problem is further complicated if the stereo endoscope is to incorporate two video sensors in its insertion portion, even with the smallest video sensors currently available. Of course, achieving stereoscopic capability by using two substantially identical optical channels offers the advantage that since the two systems are identical, their images are optically symmetrical. As used herein, the term "optically symmetrical" is intended to denote that the two systems are identical in magnification (power), orientation, the field of view (i.e., the angle encompassed by the objective lens unit), focus and optical clarity.
However, the need to maintain optical symmetry in a stereo endoscope tends to increase manufacturing cost, thereby discouraging purchase and use of stereo endoscopes. In addition, surgeons practicing minimally invasive surgery favor endoscopes with an insertion portion having a maximum outside diameter ("o.d.") of about 10 mm or smaller. Insertion portions with a larger o.d. may not be feasible or welcome for specific endoscope applications for various reasons, e.g., because of the nature of the surgical site, lack of compatibility with other instruments such as cannulas, or for cosmetic reasons or for reason of patient well-being.
The requirement of an endoscope o.d. of approximately 10 mm is easily satisfied in the case of conventional single channel endoscopes. In such case the objective lens unit or system may occupy a substantial portion of the cross-sectional area of the endoscope tube (i.e., the insertion portion). Thus, for example, the objective lens unit may have an o.d. as large as 7-8 mm in an endoscope tube having a 10 mm o.d. and a wall thickness of about 1-2 mm.
It is difficult to keep the o.d. of the insertion portion of the endoscope to about 10 mm if two objective lenses are disposed side-by-side without sacrificing image quality. Of course, the diameters of the two objective lens systems may be decreased so as to respect the 10 mm o.d. limitation, but reducing the diameters of the objective lens systems introduces other complications such as reducing image brightness and otherwise detracting from the quality of the image seen by the user.
U.S. Pat. No. 5,166,787 attempts to solve some of the design problems associated with electronic endoscopes by providing the insertion portion with two video units (each comprising an objective lens and an image sensor or recorder) that are disposed one behind the other when the endoscope is inserted into a cavity to be examined and are deployed by swinging outwardly into side-by-side relation after insertion into the cavity. However, the form of endoscope structure disclosed by U.S. Pat. No. 5,166,787 is relatively complicated and expensive to make, and its use is affected by the need to (a) deploy the video units into side-by-side position in order to make the needed observations in the body cavity and (b) move them into a tandem relationship in order to permit insertion and withdrawal of the endoscope. Moreover, the fact that at least one video unit is movable into and out of a nesting position within the endoscope housing tends to complicate provision of means for transmitting adequate light to the forward end of the endoscope. Also, as the insertion portion of the endoscope is maneuvered for viewing purposes, the movable video unit may engage an obstruction such as tissue or an organ that offers enough resistance to impede angular shifting movement of one movable video unit relative to the other unit, thereby hampering deployment into the position required to produce an acceptable stereoscopic display of the viewed surgical site.